Eraser and process for its manufacture

ABSTRACT

An eraser based on polyvinyl chloride having particles of a crosslinked polymeric natural or synthetic rubber embedded in the polyvinyl chloride structure. Such erasers have high strength and good aging resistance. Additionally, conventional softeners in the eraser do not undergo significant migration and consequently, the eraser does not smudge. A process for manufacturing the eraser is also disclosed.

This is a division of application Ser. No. 100,959, filed Dec. 6, 1979,now abandoned, which, in turn, was a division of application Ser. No.067,001, filed Aug. 16, 1979, now U.S. Pat. No. 4,335,033.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an eraser based on polyvinyl chloride (PVC)which also contains softeners, fillers, and optionally, dyes and/orother additives.

2. Description of the Prior Art

Erasers based on PVC are known, for example, from German Pat. No.932,540. Furthermore, from German Pat. No. 21 34 792, U.S. Pat. No.2,919,180 as well as U.S. Pat. No. 3,738,951, rubber erasers are knownwhose matrixes consist of several rubber components.

Each type of known erasers has inherent disadvantages which are largelycaused by the respective basic materials. For example, the previouslyknown PVC erasers have relatively little strength and their softenershave a tendency to migrate. Rubber erasers have little resistance toaging and tend to smudge when used under certain conditions.Additionally, rubber erasers can be manufactured only by relativelycomplicated procedures.

SUMMARY OF THE INVENTION

I have discovered an eraser which does not have the disadvantagesdescribed and accordingly, has a high relative strength and good agingresistance. Moreover, the softeners do not significantly migrate and theeraser does not smudge. Additionally, I have discovered a simple andextremely inexpensive method for manufacturing the eraser.

Particularly, I have discovered that such an eraser of the PVC type canbe formed from, in addition to softeners, fillers, along with optionaldyes and/or an accelerator, an unvulcanized natural or synthetic rubberin powder or pourable form, mixed in with the polyvinyl chloride or thePVC copolymer. This mixture is as extrudable as a pure PVC or a PVCcopolymer material and the eraser rubber obtained combines the desirableproperties of the previously known PVC type erasers, but not thenegative properties.

Previously, it was thought possible to produce PVC erasers only byextrusion, while rubber erasers were manufactured by a casting or arolling process. However, with the inventive mixture, it is possible forthe first time to also extrude erasers containing rubber. However, thebasic requirement for doing this is that the rubber be present inunvulcanized form as a powdered or pourable material.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In order to obtain the rubber components of the mixture in theunvulcanized state in powder or pourable form, various known methods,e.g., freeze-drying, may be used. Such "powdered rubbers" are obtainableas items of commerce. For the process of the present invention, therubber material should preferably be present in particle sizes of lessthan 50 μm.

According to the invention, the multidimensionally, crosslinkablepolymers of the rubber which, under the influence of externaltemperature, UV or cathode radiation or similar effects, form amultidimensional structure between individual molecules by crosslinking,are embedded in the softened PVC or PVC copolymer and firmly anchoredthere. The PVC preferably has a K value of about 70 and the PVCcopolymer a K value between 55 and 80. Polyvinyl chloride/polyvinylacetate may be used as the copolymer.

Rubbers or similar polymers, suitable for the purposes of the invention,include, for example:

1. natural rubber material in its various forms;

2. butadiene styrene;

3. butadiene acrylonitrile; or

4. sulfur and chlorosulfur factice.

One or several of these or similar polymers in mixture with one anotherare mixed in with the PVC plastisol compounded with the softener, as aresult of which, for example, under the action of heat, the polymerparticles crosslink and become embedded or firmly anchored in the PVCstructure. As a result, the most essential properties of the eraser areincorporated while this mixture remains extrudable.

The invention is described in the following examples. All quantitiesrefer to parts by weight.

EXAMPLE 1

In an agitator or a high-speed, high-performance mixer, PVC plastisol isprepared by stirring a softener into the powdered PVC. The polymer isstirred into this plastisol. The material so obtained is subsequentlyevacuated and mixed with the filler. This flowable mixture, referred toas "dry blend", is extruded at temperature ranges of 90/110/120/130/ and135 degrees C. The strand, emerging from the orifice of the extruder, isconveyed on a roller conveyor, cooled after about 30 seconds in a waterbath and subsequently cut into appropriate usable pieces.

EXAMPLE 2

    ______________________________________                                         41.0 parts of PVC,                                                            13.0 parts of di-2-ethylhexyl phthalate,                                      28.0 parts of filler,                                                         18.0 parts of chlorosulfur factice                                           100.0 parts                                                                   ______________________________________                                    

The above mixture is processed as described in Example 1.

EXAMPLE 3

    ______________________________________                                         40.0 parts of PVC,                                                            10.0 parts of di-2-ethylhexyl phthalate,                                      29.9 parts of filler,                                                         20.0 parts of powdered rubber (vulcanizable)                                  0.1 parts of accelerator (N,N'--diphenylthiourea)                            100.0 parts                                                                   ______________________________________                                    

The above mixture is processed as described in Example 1

EXAMPLE 4

    ______________________________________                                         35.0 parts of PVC,                                                            8.0 parts of di-n-butyl azelate                                               15.0 parts of filler                                                          42.0 parts of acrylonitrile-butadiene rubber                                 100.0 parts                                                                   ______________________________________                                    

The above mixture is processed as described in Example 1.

The material to be used as the filler, may be chalk, pumice powder,powdered quartz of the most widely differing particle size or a similarmaterial, which may be more or less abrasive, depending on the intendedarea of application. Additional materials, which may be mixed in, areoils or waxes as extrusion aids (lubricants), sulfur or similarmaterials as vulcanization aids, etc.

What is claimed is:
 1. A method for the manufacture of an eraser basedon polyvinyl chloride or polyvinyl chloride copolymer comprisingpreparing a polyvinyl chloride plastisol or polyvinyl chloride-copolymerplastisol in connection with unvulcanized synthetic or natural rubber,wherein first a pourable or flowable homogeneous substance is producedfrom mixing the polyvinyl chloride plastisol or polyvinylchloride-copolymer plastisol and the rubber, and this substance is thenfed into an extruder which has varying temperature ranges which increasefrom approximately 90° to 135° C. and from whose nozzle the cured orvulcanized eraser strand emerges and is cut into usable eraser sizepieces after passing through a cooling zone.
 2. A method for themanufacture of an eraser based on polyvinyl chloride or polyvinylchloride copolymer comprising preparing a polyvinyl chloride plastisolor polyvinyl chloride copolymer plastisol, mixing an unvulcanized rubberselected from the group consisting of natural rubber, butadiene styreneor butadiene acrylonitrile, in with said plastisol to produce a dry,flowable mixture, extruding this mixture at vulcanizing temperatures inan extruder whose temperature range increases toward the exit asfollows: 90°/110°/120°/130° and 135° C., and cutting into usable erasersize pieces after passing through a cooling zone.